We report direct measurements of poly(ethylene oxide) (PEO) mediated depletion attraction between colloids and surfaces using total internal reflection microscopy (TIRM) and in quasi-2D colloidal phase behavior using video microscopy (VM). PEO concentration dependent particle-wall depletion attraction is accurately quantified by a modified Asakura-Oosawa (AO) potential. The modified AO potential employs an equation of state, depletion length, and partition coefficient computed using results from renormalization group (RG) theory. Quasi-2D phase behavior measurements are in excellent agreement with Monte Carlo (MC) simulations using the same modified AO potential. The analytically simple modified AO depletion potential accurately captures depletion attraction for a realistic excluded volume polymer system, which suggests it can be generalized to other material systems.